A number of applications exist where an image sensor is used to scan a surface. In many such applications it is advantageous for the image sensor to be close to, or in contact with the surface.
Prior art image sensors that are close to, or in contact with the surface are typically one-dimensional. The reason for using a one-dimensional photodetector array in such image sensors is to allow the surface to be illuminated from a light source parallel to the photodetector array. In order to obtain a two-dimensional image of the surface, the surface and the photodetector array are moved relative to each other.
Applications also exist where the surface is coded using densely spaced dot patterns. The dots patterns typically encode at least a position on the surface.
One such a pattern uses dots spaced 120 microns apart. In order to achieve a minimum sampling rate of two, an image sensor is needed with a photodetector size of 60 microns, or minimum resolution of 423 pixels per inch (ppi). The dot patterns are two-dimensional patterns, and in one specific case each pattern has an extent of 100 pixels by 100 pixels.
As the two-dimensional extent of the image sensor increases to match that of the pattern to be sensed, providing illumination for the surface becomes increasingly difficult. In particular, the image sensor typically blocks light from reaching the surface underneath the centre regions of the sensor. A typical solution is to space the sensor away from the surface, but such a solution introduces further problems with regards to focussing the sensor.
A need thus exists for a two-dimensional image sensor which is as thin as possible, which has a field of view sufficient to sense at least one full dot pattern, and is self-illuminating.